The Prophet (PBUH) reported that Allah said, "I created my servants in the right religion but devils made them go astray". The Prophet (PBUH) also said, "Each child is born in a state of "Fitrah", then his parents make him a Jew, Christian or a Zoroastrian, the way an animal gives birth to a normal offspring. Have you noticed any that were born mutilated?"

On a serious note, other than outright heresy and clear actions that indicate that one is not a follower of the religion there is / cannot be such an assessment. This is recognised in Islam, and in Sufi circles it is specially reinforced that external assessments of religiosity are flawed.

There is a Mulla Nasrudin joke about this to the effect that his goat must be the most pious since it had the longest beard

Since it is impossible to access a person's internal mental life directly, what external criteria do you use to determine whether someone has submitted his will or not?

In simple terms, it is the utterance and actions of one. However only God knows the true intentions and sincerity of people. We can only see the outwardly.

Sufis go into mystical stuff and are deviated, going too far in their utterances of 'internal' matters. Some have good intentions. But from an orthodox perspective of islam, it is your speech AND actions while the internal is only known by God.

Some of the greatest exponents of Islam were Sufis or associated with Sufi Orders. These include Ghazali, Imam Hanafi, Ibn El Arabi, Ali Hujwiri, Jalaladin Rumi, Abdul Qadir Gilani, and more.

Just because there are cults, deviations, and other distortions doesn't mean the whole 'craft' so to speak is deviant. Remember Rumi's words "There is false gold only because there is such a thing as the Real..."